The present invention relates to diffraction gratings and, in particular, to a method for fabrication of low efficiency diffraction gratings and the product obtained thereby.
Diffraction gratings are useful for sampling beams of electromagnetic radiation to analyze the purity of the beam, such as the uniformity of its wavefront, and to determine the beam direction. Information from the analysis enables the beam to be better directed or to enable its wavefront quality, e.g., its coherency, phase and uniformity, to be improved.
The type of diffraction grating used for typical beam sampling applications includes a reflective surface on a substrate with one or more layers of dielectric reflective enhancing material on the reflective surface. A conventional grating, such as is illustrated in FIG. 1 herein, comprises grooves in the reflective surface with the dielectric material deposited thereon. The depth of the grooves or grating affects the efficiency of the diffraction; in general, the greater the depth, the greater the diffraction efficiency within the range of operation in sampling efficiencies of a few percent, e.g., 1% 5%, to 80%.
Such low diffraction efficiency gratings for laser beam sampling are formed by etching grooves into the metal reflector. For low power lasers, the grooves can be relatively deep, e.g., 300 Angstroms or greater. However, newer lasers have been developed with increased power and, as the laser power has increased or become more intense, the sample taken must be a smaller fraction of the total beam intensity to avoid harm to the analyzing equipment. When such a smaller fraction is taken, the required diffraction efficiency of the ratings decreases to the extent where it is less than 0.0002. Thus, to achieve such very low diffraction efficiencies, the grating groove depths must be shallower than previously made, that is, to depths of less than 150 Angstroms.
Two factors must be considered when shallower gratings are employed, viz., the control of the depth of the grooves when they reach depths of 100 Angstroms or less, and the affect of the dielectric coating over the grating. With respect to the latter, the application of a dielectric coating over the rating sometimes leads to sampling efficiencies which are difficult to predict or control, and result in gratings that are subject to anomalous effects, such as waveguiding of diffracted beams between the dielectric layers. This occurs because, in the prior approaches, the ratings are placed in the reflective surface, with the dielectric overcoat or film being placed thereon, as a conformal coat. Such an overcoat decreases the depth of the gratings in proportion to the thickness of the dielectric coating, and causes the shape of the ratings to become less distinct, both of which become even greater problems as the dielectric coating increases in thickness. The result is an increasingly poorly defined grating for sampling, that is, its corners become rounded and its depth more shallow. With regard to the former, as the depths of the grooves of the ratings are reduced in size to accommodate the higher levels of laser energy, the deleterious effect of the dielectric coating becomes greater. Furthermore, every layer in the dielectric overcoat becomes modulated by the grating, leading to complex interactions between the grating and the coatings.
In addition, the conventional processing steps for fabricating diffraction gratings create further problems. This fabrication technique comprises the steps of (1) placing a mirror surface on a substrate in a vacuum film deposition chamber, (2) removing the substrate with the mirror surface from the chamber, (3) coating the metal mirror with a photoresist, (4) exposing the photoresist with a holographic grating pattern, (5) developing the pattern into the photoresist, (6) ion etching the grating through the photoresist pattern into the metal, (7) removing the photoresist pattern with a solvent rinse, leaving the grating pattern in the metal, and (8) placing one or more layers of dielectric reflective enhancing material on the thus processed substrate with the grating pattern therein. This processing increases the likelihood of contamination and/or defects, such as film delamination and other failures.